Fly ash created from the combustion of coal is frequently used as a partial replacement for cement in concretes and mortars. Effluent gas from the combustion of coal contains mercury. This contaminant may be adsorbed by powdered activated carbon (“PAC”) injected into the flue gas stream and collected with the fly ash in a particulate removal device. For example, a brominated PAC manufactured for mercury sorption is provided by U.S. Pat. No. 6,953,494. However, when known PAC sorbents used for mercury emission control become mixed in with the fly ash from coal-fired power plants, the ash can no longer be sold for its highest-value use, namely, as a partial replacement for cement in concretes. This is because the highly-adsorptive PAC used for capturing the mercury also adsorbs the air-entraining agent chemicals (AEAs) later added to the concrete slurry to generate the air bubbles required for concrete workability and freeze-thaw capabilities.
According to the American Coal Ash Association, the US produced 65.6 million metric (MM) tons of fly ash in 2008. Replacing cement in concrete is the primary use of fly ash. About 11.5 MM tons of fly ash went to the concrete market, and about 16.0 MM tons were used in structure fills, soil modification, and other applications. Reuse of fly ash to partially substitute cement in concrete represents a major success of waste recycling in the US and has significant economic, environmental, and technical benefits.
The economic benefits of using fly ash to replace a fraction of the cement in concrete include increased revenue from the sale of the ash, reduced costs for fly ash disposal, and savings from using the ash in place of the more costly cement. Concrete performance benefits include greater resistance to chemical attack, increased strength, and improved workability. Environmental benefits include reduced greenhouse gas emissions, reduced land disposal, and reduced virgin resource use. All of these benefits are lost if fly ash compositions contain prior-art mercury sorbents beyond de minimis levels. This is doubly negative, because not only must the fly ash be disposed of rather than beneficially used, but the opportunity is missed to physically and chemically sequester the mercury from release and interactions with the environment by encasing it the concrete.
For the majority of coal-fired power plants, those without sulfur-dioxide wet scrubbers, the lowest-cost, leading-candidate technology to comply with current reduced mercury emission requirements is the injection of PAC into the flue gas in front of the plants' existing particulate controls. In this process, however, the PAC gets mixed in with the plant's collected fly ash. Because of the high surface area of typical PACs and their high adsorption capacity, if even the smallest amount gets mixed in with fly ash, the fly ash can no longer be used in concrete. The PAC adsorbs the AEAs later added to the concrete slurry. These surfactants enable incorporation of the precise amount of air bubbles needed to create the air voids required for concrete workability and freeze-thaw capabilities. For plants that could otherwise sell their ash for concrete, but now must dispose of it, this would be a big economic loss. U.S. Department of Energy National Energy Technology Laboratory analyses indicate that this deleterious by-product effect would effectively quadruple the cost of mercury reductions at some plants.
Mercury emissions from cement kilns are also increasingly recognized as a problem. PACs could similarly be injected into these exhaust gases and be collected in the particulate removal devices that separate the cement kiln dust from the exhaust gases. However, because the collected cement kiln dust would then contain AEA-adsorbent PACs, it could no longer be sold as cement for air-entrained concretes.
Others have endeavored to make carbon mercury sorbents more concrete friendly or to improve their mercury performance.
In U.S. Patent Publication No. 2003/0206843, Nelson taught that post-treatment of PAC sorbent with a sufficient amount of ozone could beneficially affect the surface properties of the sorbent enough to decrease adsorption of AEAs and render fly ashes incorporating them useful for concrete. Unfortunately, it was also found that due to the high surface area of the PAC sorbent necessary for power plant mercury control, the amount of ozone required was too great and expensive for this route to have any practical utility. To sufficiently lower the AEA interference in that disclosure, for example, Nelson taught that on the order of 1 kilogram of ozone was required per kilogram of carbon. FIGS. 10 and 11 of the patent of Chen, U.S. Pat. No. 6,890,507, indicate a similar finding.
U.S. Pat. No. 6,027,551 to Hwang teaches that unburned carbon particles from the direct combustion of coal can be separated from fly ash, post-treated with oxygen-rich gas, ozone, or nitric acid to create an improved mercury sorbent, and then injected back into the fly ash-containing gas stream to remove mercury. However, this technology involves the processing of massive amounts of fly ash to separate the unburned carbon particles, which have a reduced mercury adsorption capacity compared to commercially-manufactured PAC, followed by a separate post-treatment step. Moreover, Hwang does not sequester his sorbent mercury away with the fly ash in concretes, but instead separates the carbon from the fly ash.
Rather than post-treating carbons as Nelson, Chen, and Hwang, other methods to directly produce PAC materials have been recently proposed. Bool, in U.S. Patent Application No. 2006/0204430, rapidly mixes a very hot, highly reactive, oxygen-enriched gas stream from a burner directly with a ground or pulverized carbonaceous feedstock to quickly and directly manufacture a powdered activated char that can then be used as a mercury sorbent. The high oxygen concentration, quick and intense mixing, fine particle size, and highly-elevated temperature significantly increase the devolatilization/carbonization rate of the carbonaceous feedstock. This contrasts with traditional, far-slower methods of producing activated carbon where the devolatilization/carbonization step is gradually and separately carried out at lower temperatures on coarse granular or pelletized feedstock in a rotary kiln or on the top levels of a multi-hearth furnace, in the presence of an inert, rather than oxygen-enriched, environment. To enable the sale of power plant fly ash as a component for concrete, Bool teaches to not mix his carbon with the fly ash, but to inject the carbon after the fly ash has already been collected in a first particulate collection device. Unfortunately, the Bool sorbent production process requires unique equipment and procedures and cannot be utilized with traditional, commercial equipment or on existing activated carbon production lines.
Thus, there is a need for mercury sorbent materials that may be used for mercury sorption in gas streams without impairing the ability of fly ash present in the gas stream to be used as a partial replacement for cement in air-entrained concrete.